1. Field of Invention
The present invention relates generally to lasers. More specifically, the invention relates to edge-emitting laser structures formed by a selective oxidation process.
2. Description of the Related Art
Solid state semiconductor lasers are important devices in applications such as optoelectronic communication systems and high speed printing systems. A common laser structure is a so-called "edge-emitting laser" whose reflective mirrors are typically formed along two side edges of a semiconductor sample. Generally, formation of native oxides in a laser is an important step to achieving good electrical and optical confinement in the structure. One approach in oxide formation is commonly known as the "surface-oxidation" technique. Examples of such an approach are described in U.S. Pat. No. 5,327,488 entitled "Semiconductor Devices and Techniques For Controlled Optical Confinement" and U.S. Pat. No. 5,262,360 entitled "AlGaAs Native Oxide," both of which were invented by Holonyak et al. As discussed in these patents, under the "surface-oxidation" approach, a cap GaAs layer is placed above a thick AlGaAs layer with a high aluminum content, which is deposited above the active layer of a laser structure. Under this "surface-oxidation" approach, the surface of the sample is first patterned with silicon nitride, protecting and exposing parts of the GaAs cap layer. The exposed GaAs areas are then removed by chemical etching exposing the underlying AlGaAs layer which has a high aluminum content. The sample is then oxidized in water vapor where the oxidation in the AlGaAs layer proceeds downwards from the surface until it reaches the active layer which has a lower aluminum content. Since the active layer has a lower aluminum content, the oxidation process essentially stops when it reaches the active layer, providing electrical and optical confinement to the laser structure.
A disadvantage of the "surface-oxidation" technique is that it is sensitive to fluctuations in the fabrication process. For instance, the oxidation rate is dependent upon variations in the aluminum composition and the processing temperature. This dependency creates reproducibility and uniformity issues in the performance of the devices. In addition, this technique may not be applicable to other solid state semiconductor lasers such as vertical cavity surface emitting lasers ("VCSEL's"), which has alternate AlGaAs layers of high and low aluminum content in an upper internal reflector.
Another approach towards forming oxides is a so-called "buried-layer" oxidation approach which is described in "Lasing Characteristics of High-Performance Narrow Stripe InGaAs-GaAs Quantum-Well Lasers Confined by AlAs Native Oxide," IEEE Photonics Technology Letters, vol. 8, No. 2, p. 176 (February 1996) by Cheng et al. Under this approach, an AlAs layer is placed above and below the active layer of a laser structure. Then, grooves are etched, forming a stripe mesa structure in between the grooves. As a result of the etching, the AlAs layers sandwiching the active layer are exposed along the sidewalls of the mesa. During an oxidation process, these AlAs layers are oxidized laterally from the sidewalls of the mesa towards the center of the mesa. However, other layers in the structure remain essentially unoxidized since their aluminum content is lower. The oxidized AlAs layers reduce the effective refractive index of the regions both above and underneath them, providing lateral electrical and optical confinement to the sandwiched active layer. Another discussion regarding the "buried-layer" technique is described in "High-Performance Planar Native-Oxide Buried-Mesa Index-Guided AlGaAs-GaAs Quantum Well Heterostructure Lasers," Appl. Phys. Lett. vol. 61 (3), p. 321 (July 1992) by Smith et al.
Similar to the "surface-oxidation" technique, a key disadvantage of the "buried-layer" approach is the difficulty in controlling the amount of oxidation. Because the oxidation rate of AlAs or AlGaAs with a high aluminum content depends upon aluminum composition and process variations, any variation in aluminum composition or process parameters will be reflected by changes in the oxidation rate, which in turn creates uncertainty in the amount of oxidation. The process is relatively temperature-sensitive. Therefore, when such a technique is applied to forming lasers, the devices typically have manufacturability and yield problems.
Accordingly, there is a need for developing a laser with accurately defined and controlled oxide regions which provide electrical and optical confinement.